Security stepped up around Parliament

Measures to protect the Houses of Parliament and London against the threat of a terrorist attack were strengthened with extra police on the streets and robust defences erected.

A concrete circle surrounded the Palace of Westminster after the Metropolitan Police positioned the blocks in a bid to protect the historic building against terrorist vehicles.

There was also a more visible police presence at Heathrow airport and other key locations in the capital.

Earlier this week terror chief Osama bin Laden's deputy called on Muslims to attack the "missions" of Britain and America.

An audio tape said to have come from Ayman al-Zawahri was played on Arabic television station al-Jazeera, urging "brothers" to follow the example of the September 11 hijackers.

The new security efforts also follow a Foreign Office warning against non-essential travel to six east African countries and the suspension of British flights to Kenya.

Scotland Yard stressed the latest measures were part of continuing security considerations and there had been no specific terror threat.

The blocks were erected last night at the start of the Whitsun recess and will remain in place for the foreseeable future, parliamentary officials said.

A Scotland Yard spokesman refused to discuss what further defences were being considered and said they never revealed whether security was being increased or decreased.

He added that security at Heathrow and other high profile locations was under constant review.

But the spokesman did reveal that an extra 150 police officers were patrolling London's streets. Security at Parliament has been tight since the September 11 attacks in the US, with armed police on guard outside the main entrances.

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